CRAB NEBULA: FIVE-YEAR OBSERVATION WITH ARGO-YBJ
- Dipartimento di Fisica dell'Università di Napoli "Federico II," Complesso Universitario di Monte Sant'Angelo, via Cinthia, I-80126 Napoli (Italy)
- Dipartimento Matematica e Fisica "Ennio De Giorgi," Università del Salento, via per Arnesano, I-73100 Lecce (Italy)
- Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, P.O. Box 918, 100049 Beijing (China)
- Istituto Nazionale di Fisica Nucleare, Sezione di Roma Tre, via della Vasca Navale 84, I-00146 Roma (Italy)
- Dipartimento di Fisica dell'Università di Roma "Tor Vergata," via della Ricerca Scientifica 1, I-00133 Roma (Italy)
- Istituto Nazionale di Fisica Nucleare, Sezione di Roma Tor Vergata, via della Ricerca Scientifica 1, I-00133 Roma (Italy)
- Tibet University, 850000 Lhasa, Xizang (China)
- Istituto Nazionale di Fisica Nucleare, Sezione di Lecce, via per Arnesano, I-73100 Lecce (Italy)
- Hebei Normal University, Shijiazhuang 050016 Hebei (China)
The ARGO-YBJ air shower detector monitored the Crab Nebula gamma-ray emission from 2007 November to 2013 February. The integrated signal, consisting of ∼3.3 × 10{sup 5} events, reached the statistical significance of 21.1 standard deviations. The obtained energy spectrum in the energy range 0.3-20 TeV can be described by a power law function dN/dE = I {sub 0} (E/2 TeV){sup –α}, with a flux normalization I {sub 0} = (5.2 ± 0.2) × 10{sup –12} photons cm{sup –2} s{sup –1} TeV{sup –1} and α = 2.63 ± 0.05, corresponding to an integrated flux above 1 TeV of 1.97 × 10{sup –11} photons cm{sup –2} s{sup –1}. The systematic error is estimated to be less than 30% for the flux normalization and 0.06 for the spectral index. Assuming a power law spectrum with an exponential cutoff dN/dE = I {sub 0} (E/2 TeV){sup –α} exp (–E/E {sub cut}), the lower limit of the cutoff energy E {sub cut} is 12 TeV, at 90% confidence level. Our extended data set allows the study of the TeV emission over long timescales. Over five years, the light curve of the Crab Nebula in 200-day bins is compatible with a steady emission with a probability of 7.3 × 10{sup –2}. A correlated analysis with Fermi-LAT data over ∼4.5 yr using the light curves of the two experiments gives a Pearson correlation coefficient r = 0.56 ± 0.22. Concerning flux variations on timescales of days, a ''blind'' search for flares with a duration of 1-15 days gives no excess with a significance higher than four standard deviations. The average rate measured by ARGO-YBJ during the three most powerful flares detected by Fermi-LAT is 205 ± 91 photons day{sup –1}, consistent with the average value of 137 ± 10 day{sup –1}.
- OSTI ID:
- 22364601
- Journal Information:
- Astrophysical Journal, Vol. 798, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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